This invention relates to a method for determining when rotating stall occurs in a turbine blade of a compressor. More particularly, it concerns a method for determining when the turbine blade is subjected to an undesired operating state in the form of so-called “rotating stall”, in which the blade pass frequency of at least one compressor stage and the associated vibration energy are monitored, the normal blade pass frequency being constituted by the operating speed of rotation of the compressor multiplied by the number of blades of the stage. Incipient rotating stall in a compressor stage is indicated when, at the normal blade pass frequency of the compressor stage, the vibration energy falls below a predetermined value at the same time as, at a blade pass frequency above the normal blade pass frequency, the vibration energy rises above a predetermined value. Rotating stall in a compressor stage is indicated when, at the blade pass frequency above the normal blade pass frequency in the compressor stage, the vibration energy falls below a predetermined value at the same time as, at a blade pass frequency below the normal blade pass frequency, the vibration energy rises above a predetermined value. The invention also relates to a device for the implementation of the invention.
By the operational state of a turbine blade is meant in this context the type of load that the turbine blade is subjected to. The operational state may be, for example, normal operation, rotating stall etc.
During the operation of an axial compressor the following unfavourable forms of flow may occur, among other things:
Incipient rotating stall which is an intermediate phase, in which some of the turbine blades are about to lose some lift because of excessive underpressure, which leads to a tendency towards instability in the airflow. The cause of incipient rotating stall may be that the air approaches the turbine blade at the wrong angle. This may cause the flow to separate at the boundary layer between the turbine blade and air (boundary separation), whereby a varying flow is generated at one or more locations along the periphery of the stage.Rotating stall occurs when a first turbine blade is subjected to the condition mentioned and the airflow is deflected towards an adjacent turbine blade which is overloaded while the other adjacent turbine blade is relieved. This causes the overloaded turbine blade to be subjected to stall, whereby the first turbine blade is relieved. Thus, a rotating stall propagates along the periphery of the stage at a speed of approximately half the speed of rotation of the turbine. In rotating stall a segment of the line of turbine blades in the compressor has too low pressure across the blades, but not all the turbine blades in the line of turbine blades lose their lift. This contributes to a great cyclical load on the blades, which entails great fatigue loading and, thereby an increased risk of rupture.
When so-called “stall” occurs, the segment of reduced air pressure has propagated to the entire line of turbine blades, which then loses the lift and thereby the ability to pump gas across the stage.
Surge occurs when one or more stages are in a state of stall and cannot transport gas from one stage to the other. Then a back and forward flow of air normally occurs in the compressor. This flow is caused by the turbine blades alternatingly establishing flow but losing it an instant later. As long as the air does not flow through the compressor and is not replaced, heat builds up in the air. The temperature quickly becomes very high, and will normally overheat affected components inside the compressor and at worst melt down turbine blades and seals.
For reasons of maintenance it is important to be able to estimate the remaining lifetime of the turbine blades. Early methods of estimation were based exclusively on operating time, the lifetime of a turbine blade, for example, being set to an operating time, during which, with reasonable certainty, satisfactory functioning could be expected regardless of the load the turbine blade was subjected to during the operating time.
Obviously, such relatively simple lifetime estimation led to excessively frequent maintenance intervals and consequently also replacement of turbine blades which had been subjected to relatively low strains during their operating time. Prior art now comprises lifetime estimation methods that are to some extent quite complicated, in which, in addition to operating time, parameters such as power loading, failures in both the component monitored and adjacent components, wear, and also faults in the measuring equipment used to measure the loads, are taken into account.
Rotating stall in a multi-stage axial compressor may result in overloading of the turbine blade with subsequent damage and compressor breakdown, without the condition being detectable by means of prior art equipment and methods.
According to the prior art a compressor is monitored by means of performance measurements. The measured values resulting from the measuring form part of the input values in a lifetime estimation computer program. The measurements are compared with anticipated values, the anticipated lifetime of the component in question or the entire turbine being affected by whether the measured value is greater or smaller than an anticipated value. However, this form of monitoring is not designed to allow determination of which compressor stage is being subjected to stall.
Norwegian patent application 20023609 discloses a method for detecting rotating stall. The method according to this application does not take into account new knowledge which has emerged during further development work.
Other methods for monitoring turbine blades are also known. U.S. Pat. No. 5,097,711 discloses a system for monitoring vibrations in covered turbine blades by means of eddy current induction in the covers of the turbine blades. The method is unsuitable for uncovered turbine blades.
EP 465696 describes a method of monitoring turbine blades by means of resonant frequencies. A differential Doppler signal is used, obtained by measuring at least at two different angular velocities.
GB 2318873 discloses a method for measuring vibrations in turbine blades, in which a pressure sensor is used on the inside of the turbine casing and a strain gauge which is mounted on the rotor blade 4. This method cannot be used in normal operation of a turbine, but only during testing, because the strain gauge and adjacent measuring equipment will not be able to resist the temperatures that may occur. Of these, only NO 20023609 addresses the discovery of rotating stall.